In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), oil country tubular goods for casings, tubings or the like, which are steel pipes and are sequentially connected to one another by threaded joints, are used. Generally, threaded joints for steel pipes for such use are classified into two types: coupling-type joints and integral-type joints.
A coupling-type threaded joint is constituted by a pair of tubular goods that are to be connected to each other, of which one is a steel pipe and the other is a coupling. In this case, the steel pipe is provided with a male threaded portion formed on the outer periphery at each end thereof, and the coupling is provided with a female threaded portion formed on the inner periphery at each end thereof. The male threaded portion of the steel pipe is screwed into the female threaded portion of the coupling, thereby making up a joint and connecting them. An integral-type threaded joint is constituted by a pair of steel pipes as tubular goods that are to be connected to each other, without a separate coupling being used. In this case, each steel pipe is provided with a male threaded portion formed on the outer periphery at one end thereof and a female threaded portion formed on the inner periphery at the other end thereof. The male threaded portion of one of the steel pipes is screwed into the female threaded portion of the other of the steel pipes, thereby making up a joint and connecting them.
In general, the joint portion at the tubular end where a male threaded portion is formed is referred to as a pin because it includes an element that is inserted into a female threaded portion. On the other hand, the joint portion at the tubular end where a female threaded portion is faulted is referred to as a box because it includes an element that receives a male threaded portion. A pin and a box both have a tubular shape because they are constituted by end portions of tubular goods.
FIG. 1 is a sectional view of a threaded joint for steel pipes, showing an example of its overall configuration. The threaded joint illustrated in FIG. 1 is a coupling-type threaded joint and is constructed of a pin 10 and a box 20.
The pin 10 includes, in order from a free end of the pin toward the tubular body, a shoulder surface 17, a seal surface 16, and a male threaded portion 11. The seal surface 16 is a tapered surface. To be exact, the seal surface 16 is a surface constituted by the peripheral surface of a truncated cone having a diameter decreasing toward the end, or a surface constituted by a combination of the peripheral surface of the truncated cone and the peripheral surface of a solid of revolution that can be obtained by rotating a curved line such as an arc about the pipe axis CL. The shoulder surface 17 is an annular surface extending radially substantially perpendicular to the pipe axis CL. To be exact, it is a slightly inclined surface with the outer circumferential side being closer to the end of the pin 10.
The box 20 includes, in order from the body of the box toward an end of the box, a shoulder surface 27, a seal surface 26, and a female threaded portion 21. The shoulder surface 27, the seal surface 26, and the female threaded portion 21 are located so as to correspond to the shoulder surface 17, the seal surface 16, and the male threaded portion 11 of the pin 10. The male threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20 are tapered threaded portions with trapezoidal threads that mate with each other.
The male threaded portion 11 and the female threaded portion 21 are threadedly engageable with each other, and in a made-up state, they mate in intimate contact with each other and have an interference fit. The seal surfaces 16, 26 are brought into contact with each other by the screwing of the pin 10, and in a made-up state, they mate in intimate contact with each other to have an interference fit, thereby forming a seal therebetween with metal-to-metal contact. The shoulder surfaces 17, 27 are brought into contact and pressed against each other by the screwing of the pin 10 onto the box 20, and serve as stoppers for restricting the screwing of the pin 10. Furthermore, in a made-up state, the shoulder surfaces 17, 27 serve to impart, to the male threaded portion 11 of the pin 10, a load in a direction opposite (backward) to the screwing direction (forward), i.e., so-called thread tightening axial force.
With a threaded joint having this configuration, good sealing performance is ensured because of the seal provided by the mating and intimate contact between the seal surfaces 16, 26.
In recent years, oil well environments have increasingly become deep-underground or ultra deep-water environments, and accordingly have become harsh environments with high temperatures, high pressures, and high corrosivity. For such harsh environments, steel pipes of the heavy wall type are mostly used. A threaded joint for connecting such steel pipes is required to have high joint strength such as resistance to tensile forces, resistance to compressive forces, and the like, and in addition, required to provide excellent sealing performance against internal pressure and external pressure.
One method for enhancing the sealing performance of a threaded joint is to generate high contact pressure between the seal surfaces. Conventionally, in order to increase contact pressure between the seal surfaces, the technique of increasing the interference fit between the seal surfaces is utilized. In addition, in order to prevent the mating and intimate contact of the threads from causing a decrease in contact pressure between the seal surfaces, the technique of relieving the mating and intimate contact of the threads exclusively in regions near the seal surfaces is utilized (see, for example, U.S. Pat. No. 2,062,407 (Patent Literature 1), Japanese Patent Application Publication No. H02-80886 (Patent Literature 2), Japanese Patent Application Publication No. S62-196488 (Patent Literature 3), and Japanese Patent Application Publication No. H10-89555 (Patent Literature 4)).
FIG. 2 is a sectional view of a conventional threaded joint for steel pipes disclosed in Patent Literatures 1 and 2, showing a configuration of regions near its seal surfaces. In the conventional threaded joint shown in FIG. 2, in a made-up state, the male threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20 mate in intimate contact with each other, and the load flanks 15 of the male threaded portion 11 are in contact with the load flanks 24 of the female threaded portion 21 and receive the axial tightening force while the roots 13 of the male threaded portion 11 are in contact with the crests 22 of the female threaded portion 21. However, in regions near the seal surfaces 16, 26 in the threaded portions, clearances are provided between the roots 13 of the male threaded portion 11 and the crests 22 of the female threaded portion 21, so that the mating and intimate contact of the threads is relieved therein.
The load flank 15 of the male threaded portion 11 as referred to herein is the flank, of the leading and trailing flanks that constitute each thread, which is on the opposite side from the stabbing flank 14, which is in a leading position in the screwing of the male threaded portion 11 into the female threaded portion 21. The load flank 24 of the female threaded portion 21 is the flank, of the leading and trailing flanks that constitute each thread, which faces the load flank 15 of the male threaded portion 11.
In the threaded joint shown in FIG. 2, the mating and intimate contact of the threads is relieved in regions near the seal surfaces 16, 26 in the threaded portions, and therefore, when internal pressure is applied, the region near the seal surface 16 in the threaded portion of the pin 10 is expanded radially outward from the inside to cause enlargement of the diameter, whereby the contact pressure between the seal surfaces 16, 26 is amplified.
It is to be noted that, in the conventional threaded joint shown in FIG. 2, the flank angle θ of the load flanks 15 and the load flanks 24 is greater than 0 degrees. The flank angle θ refers to the angle formed by a flank with respect to a plane perpendicular to the pipe axis CL. Herein, when referring to the flank angle of load flanks, clockwise angles are designated as positive angles, and conversely, when referring to the flank angle of stabbing flanks, counterclockwise angles are designated as positive angles. When the load flank angle θ is greater than 0 degrees, application of internal pressure causes reaction force at the load flanks 15 of the pin 10 acting in a direction to contract the pin 10 radially inward from the load flank 24 of the box 20. As a result, the radially outward expansion of the pin 10 is not sufficiently caused, and therefore the amplification of the contact pressure between the seal surfaces 16, 26 cannot be satisfactorily achieved when internal pressure is applied.
FIG. 3 is a sectional view of a conventional threaded joint for steel pipes disclosed in Patent Literature 3, showing a configuration of regions near its seal surfaces. In the conventional threaded joint shown in FIG. 3 as well, in regions near the seal surfaces 16, 26 in the threaded portions, clearances are provided between the roots 13 of the male threaded portion 11 and the crests 22 of the female threaded portion 21, so that the mating and intimate contact of the threads is relieved therein. Its load flanks 15, 24 have a flank angle θ of less than 0 degrees.
In the threaded joint shown in FIG. 3, because of the load flank angle θ that is less than 0 degrees, application of internal pressure does not cause reaction force at the load flanks 15 of the pin 10 acting in a direction to contract the pin 10 radially inward. As a result, the radially outward expansion of the pin 10 is sufficiently caused, and therefore amplification of contact pressure between the seal surfaces 16, 26 can be achieved when internal pressure is applied.
In the threaded joint shown in FIG. 3, it is to be noted that, in regions near the seal surfaces 16, 26 in the threaded portions, the height of crests 12 in the male threaded portion 11 of the pin 10 decreases toward the end of the pin 10 with a steep taper angle, so that the thread height is sharply decreased toward the end. Because of this, the stiffness of the pin 10 is reduced. This causes a decrease in the deformation resistance of the pin 10 against external pressure, which results in reduced contact pressure between the seal surfaces 16, 26 when external pressure is applied. In addition, the low thread height of the male threaded portion 11 in the region near the end of the pin 10 results in reduced thread mating at the time of insertion of the pin 10 into the box 20, and this causes increased eccentricity of the pin 10. Thus, at the start of screwing of the pin 10 onto the box 20, the thread corner regions of the male threaded portion 11 of the pin 10 locally contact the female threaded portion 21 of the box 20. In the regions where such local contact occurs, the contact pressure is increased and therefore galling is more likely to occur.
Furthermore, the threaded joint shown in FIG. 3 is configured such that the roots 23 of the female threaded portion 21 of the box 20 have different taper angles. This configuration requires a complex manufacturing process, which leads to a longer manufacturing time and shorter tool life, and therefore a further problem of increased manufacturing costs arises.
The threaded joints shown in FIGS. 2 and 3 are both configured to relieve the mating and intimate contact of the threads in regions near the seal surfaces in the threaded portions, by providing clearances between the roots of the male threaded portion and the crests of the female threaded portion. As another technique for relieving the mating and intimate contact of the threads in regions near the seal surfaces, Patent Literature 4 discloses a technique of providing a circumferential groove between the female threaded portion of the box and the seal surface thereof.
However, in the threaded joint disclosed in Patent Literature 4, thread mating is reduced in regions near the seal surfaces because of the circumferential groove provided in the box. Because of this, when external pressure is applied, radial contraction of the pin easily occurs, resulting in a decrease in sealing performance against external pressure.